The antibody responses of aged animals and humans may change in the magnitude and/or in the structure of antibody molecules. The long-term objective of this project is to elucidate the mechanisms of those immunological changes, and their effects on the ability of aged individuals to resist infections, using an experimental model of mouse antibody response against S. pneumoniae strain R36a (Pn). The Pn-antibody response, which is directed against the immunodominant bacterial epitope, phosphorylcholine (PC), protects the mice against lethal infection with pneumococci. The PC antibody molecules produced by young/adult mice (2-6 mo. old) are encoded by a single combination of V (D) J genetic segments designated as T15 genes. Surprisingly, the antibody produced by aged (greater than 20 mo. old) mice may be quite robust, but it appears to be encoded by different germline Ig genes. The first proposed aim is to determine as to how many different V gene families encode the H and L chains of Pc-specific hybridomas Ab generated from aged mice. Selected VH (V-D-J) regions will be sequenced to assess the extent of somatic mutations, in comparison with similar genes from young mice. The effects of genetic shift on the specificity and anti-pneumococcal activity of aged PC-antibody will be determined in both active and passive protection experiment. Subsequent aims are on the mechanisms of age-related antibody repertoire shift. An adoptive transfer of purified lymphocytes will be used to determine whether the aged pre-B cells develop into different PC-reactive clones, alone or whether the shift is influenced by the aged T cells. The competence of aged T cells to regulate the magnitude and the diversity of antibody response to PC antigens will be studied both in vitro and in athymic mice reconstituted with T cell subsets from young and aged donors. The proposed studies will determine whether (a) the aged CD4- cells fail to drive the formation of germinal centers as well as the somatic diversification of the antibody repertoire, and (b) the magnitude of PC response in aged mice is determined by the germline make-up of the host via a mechanism related to T cells.